Solid Stete vs Tube Rectifier

[Enzo]

The purpose of the standby switch is to silence the amp without turning it off. Leaving standby means you can instantly make sound without waiting a few seconds for warmup.

Yes, but that poor rectifier tube - it has high voltage on its plates instantly before it gets warm. How come everyone worries about "cathode stripping' on their power tubes, which are not in any real danger of it, but no one cares about the rectifier tube?

[Chuck H]

Because the rectifier tube doesn't make the sound, the power tubes do. No one cares about the help, only the aristocracy.

Seriously though, it's been discussed here many times but... Is it really true that there is nothing wrong with applying HV to the tubes before heating them up??? And if there is a problem how can I set up an automatic delay in HV? Most tubes spec less that twenty seconds to warm up. Assuming this could fluctuate I think a thirty second delay in HV should be fine. And why would a tube have warm up time specs? Is this just so we all know how long it will be before the tube makes sound? Or is this so we know how long to warm them up before using them? There is A LOT of contrary info either way. Much of it backed with some degree of tech prowess or consideration. So what's the real deal???

Chuck

[Enzo]

The problem - as I see it anyway - is that people become aware of some phenomenon (skin effect, cathode stripping, whatever), and without considering the context start to apply it. And furthermore, people find various things in correlation and decide that they are therefore related - a logical fallacy. People like to fill in the blanks: here is this, and I know about that, so there must be some link between them.

Tube specs include warm up times because tubes have to work with other tubes. COntrolled warm ups are especially important in series string heater arrangements. But an engineer also wanted to know how long it was going to take to have a circuit come into function. Who cares how a Fender Champ warms up? But a guy designing TVs at RCA didn;t want the set picking up wrong channels the furst few minutes, not did he want a loud BWAAAROOOMZWEEPPP coming out the speaker before it settled down. If your radar installation ran 24 hours a day, who cared how it warmed up, but consumer goods were a different matter. When I went off to college, I had a plain old All American Five clock radio, and dad's TV had tubes.

Most of the tubes in stuff are conducting whether there is signal or not - all that class A stuff like the preamp tubes. SO warm up times are not about waiting to use it. As far as that 12ax7 is concerned, it doesn;t know if you are playing through it or not.

Oh, look what I just found: Giant Fake Vacuum Tube

Small amps don;t even have standby switches. Larger amps like a Fender Twin as you point out are often turn completely on from the start and they last and last and last. And ther are plenty of examples of standby switches that do not remove the voltage from the tubes. They might short opposing power tube grids together or short the PI outputs together. They can lift cathodes. And most larger Peavey amps leave the plates hot but open the screen node for standby.

The people who need to be concerned about cathode stripping are the guys maintaining those 10,000 watt or 50,000 watt radio transmitters, or the guys maintaining large industrial induction heaters and the like. Guys with water cooled tubes or at leat forced air cooling finned tubes.

Until the cathode gets up to a certain temperature, it won;t be emitting anyway, and in that cold moment, to damage the tube the rest of the system would have to have enough balls to TEAR the coating off the poor cathode. 350v on your 6V6 just is not going to do that. 4kV with all the current in the world might be a different story.
File:Hp vacuum tube.jpg - Wikipedia, the free encyclopedia

[Chuck H]

+++
Thank you. FWIW I always suspected that cathode stripping was a phenomenon limited to much higher performance systems than guitar amps. This based on my readings here. As a result some of my early frankenstein amps don't even have a standby switch. But I also figured that even though cathode stripping was only markedly notable in higher performance circuits it must be a consideration, albiet a smaller one, in more pedestrian gear too, to a smaller, maybe even negligable extent, but still a factor. I've never really worked on a delayed B+ for this reason and I suppose I won't ever bother unless I hear a really convincing argument to the contrary.

[Enzo]

Everything is a factor at some level. Indy cars have a big airfoil above the rear to provide down-thrust for improved traction. AT speeds above 140mph, it really pays off. Now I bet if we welded one on your mom's car it would provide an extra ounce of down force at 30mph on the way to the store. But would anyone feel that?

Oh wait, they already did that. On the rear of some of those ugly Pontiac Aztec SUV things, ther is a little rear fin 2/3 of the way up the back. Clearly totally cosmetic, more useful as a place to park your gloves while fishing for the keys, but no doubt a lab scale could measure some effect on the car.

Don;t know if you know Bob Pease, he's a famous engineer for National Semiconductor. Writes about analog all the time. Smart guy. He tells stories about design engineers asking NAt Semi about their parts for applications they design. One recurring theme is engineers seing a temp stability spec and thinkming they can extrapolate beyond the specified range. They can't.

We can extrapolate from some 10,000 watt tube down to a 20 watt tube, but there is no guarantee of a linear scaling. We have no reason to assume there will be 20/10,000 of the full effect. More likely you drop down a curve until a threshold is hit, and nothing happens at all below it.

[defaced]

One recurring theme is engineers seing a temp stability spec and thinkming they can extrapolate beyond the specified range. They can't.

I seem to remember a certain space shuttle blowing up when I was a kid because someone did that. Extrapolation is a painfully dangerous endeavor.

That said, a delayed turn on is easy to do. A monstable one shot 555 timer with an inverting stage after it would be a way to do it. And MCU another way. I'm sure there's a CMOS/TTL chip(s) that could be made to do it. Any one of these controls a series relay , or a power MOSFET, or a... with the B+ line that must be on with the standby switch to provide B+ to the circuit.

[Chuck H]

+++ and thank you. While I do have an aversion to sticking any uncommon SS devices in my eyelet board tube amps I was going to make an exception for this B+ delay. I was going to use a timer and a MOSFET. What I was saying is that I hadn't worked on it yet. And I was wondering about anyone elses ideas, pro's and con's, etc. Now I don't suppose I'll bother. Enzo's theory of "below a certain threshold nothing happens" was the first thing I considered after reading his first post. There's a lot of that kind of thing in electronic components. I don't imagine the nature of the cathode coating or material is much different in high perfomance tubes like transmitters and such than it is in the tubes we use. If that material doesn't start to "boil" until the voltage is extreme then a few hundred volts is proportionally little more than if the tube was still in the box.

Chuck

[defaced]

I've not done it either. I guess that's because I only build amps for me and well I know that it doesn't make a difference even though I prefer to let the amp warm up before I play (and inrush current kinda forces me to...)

But while we're talking about cathode behavior, in my head, I kinda think of a cathode as a temperature dependent insulator (or conductor, depends on how you think of your half glass of water). At low temp and low voltage, nothing happens. At high temp and low voltage we get current flow. At low temp and high voltage we get cathode stripping (and current flow). And at high temp and high voltage we get current flow. High voltage can rip off the electrons in a destructible manner but the application of heat makes the electrons "free" and "replaceable" thus the reaction is stable/repeatable/whatever. Forced conduction vs free conduction.

[Chuck H]

Right. The deciding factor is how bad is the cathode stripping effect in the common usage of guitar amps. I'm not really sure. But everything I've seen so far in real world experience has been indicative of it being negligable. This is supported by what I've read on the subject too. I'll sleep a little easier tonight.

Chuck

[Enzo]

Grab the RCA manual and read the chapter on how tubes work. Or a nice write up in chapter 1 of the RDH.

The Cathode is coated, and has to come up to something like 777 degress C. It emits a cloud of electrons and then... amplification happens. The actual conducted current is supposed to be less than the space charge. And.. oh just read it.

Those huge transmitting tubes are more likely to be thoriated tungsten cathodes, and direct heated, and operating at considerably higher temperature. COntrast to the oxide coated nickel cathode in one of our tubes.

[Merlinb]

True cathode stripping happens when the cathode is exposed to field strengths in excess of 4 million volts per metre. A receiving tube will barely manage half a million volts per metre, even under the most extreme start up conditions- stripping is physically impossible.

Cathode sputtering (attack by gas ions) is only a concern when the gas content is high, i.e., in gas tubes. In receiving valves the gas content is so low that a little sputtering -even if it did happen at start up- would have no effect on normal cathode operation. There is simply too much free barium available for a few gas ions to even make a dent in the emission.

[tubeswell]

Another advantage of tube rectifiers is that you can design your amp so you can change tube rectifiers (and B+) to run different output tubes etc with different rectifiers. eg a 5Y3 with 2 x 6V6s or a 5AR4 with 2 x 6L6s (PT current draw, OT load resistances, and all other stuff taken into consideration of course) - whether its a SE or a PP amp.

[Merlinb]

Another advantage of tube rectifiers is that you can design your amp so you can change tube rectifiers (and B+) to run different output tubes etc with different rectifiers.

Or indeed use one switch and some cheap power resistors....

[tubeswell]

Or indeed use one switch and some cheap power resistors....

Advantage conceded

[Diablo]

Or indeed use one switch and some cheap power resistors....

Yes, but your amp will never please the folks that claim that a NOS Mullard GZ34 sounds better than any other rectifier.......